Radio frequency identification (RFID) tags are now widely used to mark, inventory and track various products. RFID tags generally transmit to a handheld or robotically controlled reader device a radio frequency signal that includes product information. RFID tags generally include an integrated circuit for storing and processing information, a transceiver for transmitting and receiving RF signals, and an antenna. Some RFID tags are active RFID tags and include their own battery power source. Passive RFID tags do not have their own power source and require receiving a power signal from a reader device to operate. For interrogating passive RFID tags, a reader generally transmits a continuous wave (CW) or modulated radio frequency (RF) signal to a tag. The tag receives the signal, and responds by modulating the signal and then “backscattering” an information response signal to the reader. The reader receives the response signal from the tag, and the response signal is demodulated, decoded and further processed.
Amplitude modulation (AM) present on the carrier wave transmitted from an RFID transmitter while a tag is backscattering appears as noise at the receiver. Sideband signals that backscattering produce are generally many orders of magnitude below an original transmitter power. By the time such sideband signals reach the reader device, they are even further orders of magnitude lower. The receiver employs a directional device to separate the incoming backscatter from the outgoing CW energy; however, any energy reflected back from an antenna of the reader device is summed with incoming backscatter signals. If the outgoing energy has sideband energy within it, such carrier sideband energy can obscure the sideband energy returning from the tags. Any AM noise on the carrier wave from the reader device can have this effect, and can thus limit the range of an RFID system.
Further, such noise is primarily proportional to the return from the transmitting antenna and generally cannot be mitigated by a current echo or carrier cancellation scheme. Various sources can cause such unwanted AM noise, including for example power supply ripple, power supply load pulling, local near field pickup, thermal noise in the transmitter amplifier chain, dynamic load changes imposed on the antenna by the environment, and digital noise.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.